Tailless aircraft provide certain advantages over conventional aircraft such as improved aerodynamic efficiency due to reduced aerodynamic drag as well as reduced weight and size. Other advantages of tailless aircraft include low radar visibility due, at least in part, to the geometric configuration of the aircraft. As is suggested by the name, tailless aircraft lack certain control surface arrangements typical of conventional aircraft such as aft-fuselage mounted tail sections for carrying a conventional elevator and/or stabilizer or a conventional tail fin and rudder.
In the absence of such conventional tail sections, tailless aircraft may employ alternative control surfaces mounted on an aft portion of the wing in order to provide direction control to the aircraft. Included among the types of control forces required for an aircraft is yaw control. More specifically, yaw control or the ability to generate a yawing moment about the aircraft is typically desired for efficient turning control during flight.
In order to generate the necessary yaw moment required for certain turning maneuvers, some tailless aircraft may employ spoilers and/or split ailerons (i.e., spliterons) on the aircraft wing. Spoilers, spliterons and other alternative control surfaces may also be employed for speed control of tailless aircraft. Although the employment of spoilers and other devices has been effective in providing yaw control on tailless aircraft, the use of such devices presents certain drawbacks and deficiencies which detract from their overall utility for yaw control.
For example, the forces that are required to actuate a spoiler or a spliteron control surface are typically directed against the force of the air stream that is passing over the wing. As such, the actuating mechanisms for spoilers and spliterons themselves must be capable of applying the relatively large deployment forces and are therefore typically large, bulky, complex, and heavy. In addition, such actuating mechanisms may consume large amounts of energy for deployment of the control surfaces at certain flight conditions.
A further drawback associated with spliterons is related to the relatively thin cross-sectional profile at the trailing edge of the wing where spliterons are typically located. Because of the relatively thin profile, packaging of the actuating mechanism for the spliteron presents design challenges. The use of spoilers presents other inherent drawbacks. For example, for spoilers that are mounted at a mid-chord position on the wing, the deployed spoiler may generate a disruption or separation in the airflow passing over the wing. In this regard, spoilers may produce significant interference with flaps, ailerons and other downstream control surfaces due to the creation of separated flow in their wake.
In addition, the separated flow produced by the spoiler diminishes the downstream control surface effectiveness and, furthermore, may result in an increase in buffet loads on the aircraft. As a result, tailless aircraft which employ spoilers for yaw control may require trailing edge devices having increased surface area as compensation for their reduced effectiveness in the wake of the deployed spoiler. Unfortunately, an increase in the surface area of the trailing edge device correlates to an increase in mass which, in turn, translates to an increase in the structural loads that are imposed upon the airframe.
Thrust vector control is another mechanism that has been employed on tailless aircraft as a means for imparting yaw moment or directional control of the aircraft via the propulsion unit or units. Thrust vector control mechanisms requires the use of thrust vectoring devices which, in some arrangements, typically necessitates the use of independently controllable throttles on two separate propulsion units. Alternatively, thrust vector control may be implemented with the appropriate ducting from a single propulsion unit.
Unfortunately, arranging the propulsion system to provide the aircraft with yaw control imposes several inherent drawbacks. For example, thrust vector control is throttle-dependent such that any achievable yaw control is necessarily proportional to the propulsive thrust capabilities. As such, in the event of engine failure or low engine thrust output, ineffective flight control or yaw control may result at critical times such as during low throttle operations typical of approaches and landings. Furthermore, the implementation of thrust vector control on any aircraft is typically heavy, complex and requires the use of hardware that is capable of withstanding high engine temperatures. A further drawback associated with thrust vector control is that some of the components may be located far aft on the aircraft such that ballast (i.e., dead weight) must be added in order to maintain the vehicle's center of gravity within an acceptable range.
A further drawback associated with the use of spoilers and/or spliterons on tailless aircraft is related to the separated flow over the wing that occurs at high angles of attack. Although such control surfaces may be effective in imparting the necessary yaw, pitch and/or roll control during most flight conditions, such control surfaces may lose their effectiveness due to separation of the flow over the wing at high angles of attack. Furthermore, the extent of the separated flow tends to increase near the trailing edge where the control surfaces are typically located. In this regard, these control surfaces may be rendered ineffective for yaw and/or speed control during high angle of attack conditions.
As can be seen, there exists a need in the art for a system and method for imparting yaw control to an aircraft which is effective through a wide range of flight conditions. In this regard, there exists a need in the art for a system and method of yaw control of an aircraft that is effective at high angles of attack. Furthermore, there exists a need in the art for a yaw control mechanism which minimizes the amount of downstream wing surface area affected by separated flow in the wake of the deployed mechanism.
In addition, there exists a need in the art for a yaw control mechanism that is deployable with minimal actuation forces and which operates with increased yaw efficiency relative to conventional control surfaces. In this regard, there exists a need in the art for a system which minimizes and reduces buffet loads and associated structural airframe requirements as a result in the minimization of separated flow. Finally, there exists a need in the art for a yaw control mechanism which is of simple construction, low cost and low weight.